The invention relates to flicker compensation for cameras, more specifically to flicker compensation for high-speed cameras such as slow-motion cameras operating at an increased field frequency.
JP-A-62/123,880 discloses a fluorescent lamp flicker correction circuit for a solid-state television camera. The magnitude of video signals is adjusted to lower flicker of a fluorescent lamp when a CCD element is used by adjusting the gain of a gain control circuit automatically according to the magnitude of video signals of each field. More specifically, video signals applied to an input terminal are applied to the gain control circuit and at the same time sent to a switching circuit. There, signals of one field are portioned to three signal lines including respective low-pass filters having the same characteristic to obtain three equalized signals which are switched successively by a switch. At the same time, they are equalized by an equalizing circuit. A divider divides the output from the switch by the output of the equalizing circuit to obtain gain control signals for the gain control circuit.
U.S. Pat. No. 5,272,539 discloses another video camera with flicker cancellation. A flicker cancelling loop has a flicker detector and detects a flicker component level in the image pick-up signal and controls the gain of an AGC amplifier and/or an opening of an iris so that the flicker component level is cancelled.
U.S. Pat. No. 5,293,238 discloses a television camera in which flicker which occurs when the television camera images an object under illumination of a flickering light source, such a fluorescent lamp, is minimized without causing sensitivity of a photo-electric conversion element and hence the camera to be degraded under a non-flickering light source. The television camera includes an automatic iris control device, an automatic gain control circuit, a micro-computer, a photo-electric conversion element, and an integration circuit for integrating an output signal of the photo-electric conversion element for each field period. The micro-computer sets a signal a signal storage time of the photo-electric conversion element to a value by which flicker noise can be restricted when flicker is detected on the basis of a change of an output signal of the integration circuit, and sets it to another value when a change of light source is detected on the basis of a change of iris value and gain. The integration circuit functions to accumulate signal for every field period and to sample/hold the integrated signal. The determination of flicker can be done by storing the input signal from the integration circuit for at least 1 field period, sequentially comparing it with a stored input signal for an immediately preceding field, and checking regularity.
The light output of artificial lighting from AC power sources is not constant in time. Especially FL and HMI light sources have a strong AC component. The cycle time of this effect is {fraction (1/100)} sec for 50 Hz power sources and {fraction (1/120)} sec for 60 Hz power.
Modern CCD cameras have a controlled exposure time. To avoid beat frequencies in the exposed picture in artificial light situations the camera exposure time can be chosen to integrate exactly one cycle time of the artificial light source used.
The choice of 50 Hz field frequency in countries with 50 Hz mains supply and 60 Hz field frequency in countries with 60 Hz mains supply was also meant to avoid beat frequencies between AC power and cameras. Even with exposure control switched to nominal, interference of the AC mains frequency with the field frequency only results in a beat frequency in the video signal at low frequencies (fmainsxe2x88x92ffield).
The above-mentioned prior art does not address the specific problems associated with slow-motion cameras operating at a high speed. For slow-motion applications cameras are used with a field frequency higher than 50 or 60 Hz. Typically slow-motion cameras work at field frequencies N times higher than the broadcast field frequency, with N=3 as the most commonly used (150 or 180 fields/sec). The signal of such cameras can be recorded, and the sequence can be played back at the normal system speed of the broadcast system, resulting in a time expansion of N (=slow-motion).
The maximum exposure time of the camera is less than one cycle period of the artificial light, and each 2 periods of the light is sampled in N fields. This means that on light with strong AC content the occurrence of beat frequencies in the camera output signal will be unavoidable. The AC content of the light source is sampled in N phases, resulting in a repeating beat pattern over every N field periods. If the relationship between field frequency and mains frequency is exact, a fixed cycle is the result, otherwise phase shifts will cause the pattern to change slowly, as the phase of the field sampling with regard to the light is changed. The disturbance of the signal then can be characterized as a pattern over N fields, slowly changing in time. FIG. 1 illustrates this. The horizontal axis represents time T in seconds, while the vertical axis represents the amplitude A. In FIG. 1, curve 1 represents the light output of a 50 Hz AC light source, while the rectangularly shaped curve 2 represents the average of curve 1 over one field period of a high speed camera having a field frequency of about 3xc3x9750 Hz. The most annoying effect is the fast change from field to field. The slow change in mean video level and field pattern is secondary.
Present slow-motion cameras have no means to correct for the effects described.
It is, inter alia, an object of the invention to provide improved slow-motion cameras. To this end, a first aspect of the invention provides a method and a device as defined by claims 1 and 3. A second aspect of the invention provides a slow-motion camera as defined by claim 4. An advantageous embodiment is defined in dependent claim 2.
A primary aspect of the invention provides a method of compensating an image signal having a field frequency of substantially N times a mains frequency, in which Nxe2x89xa72, for AC light source induced fluctuations, the image signal and an average signal representing an average image signal content over a given time period are arithmetically processed to obtain a corrected signal, in which at least Nxe2x88x921 differences between at least Nxe2x88x921 respective phases of the corrected signal on the one hand, and a selected phase (ph2) other than the Nxe2x88x921 phases of the corrected signal, or an average over N fields of the image signal, on the other hand, are integrated, and in which at least Nxe2x88x921 integrated differences are processed to obtain a correction factor for the image signal.